The present invention generally relates to embedded resistor devices and, more particularly, to embedded resistor devices with an improved radio frequency (RF) performance.
Resistors have been widely used in circuits such as current-limiting circuits, voltage regulators and termination impedance controllers. Some resistors may be mounted on circuit boards utilizing a relatively complicated process such as the surface mount technique (SMT), which may occupy large area on the circuit boards. To reduce the dimensions of resistors, embedded resistor devices have been developed, which may be formed by resistor-coating techniques. FIG. 1 shows a cross-sectional view of a conventional embedded resistor device 100. As illustrated in FIG. 1, the embedded resistor device 100 may include a resistor material 102 coated on a dielectric layer 104, which may be formed on a ground plane 106. The resistor material 102 may include one end coupled to the ground plane 106, and the other end coupled to a conductor as a terminal of the single-port, embedded resistor device 100. However, because errors may occur during circuit-printing on a circuit board, and defects may exist in a coating material, a calibration process may be required for adjusting the resistance of the embedded resistor device 100. The calibration process may be performed with a laser machine and may increase the manufacturing cost.
Furthermore, because the embedded resistor device 100 may include different kinds of materials in different layers manufactured by different processes, parasitic effects may occur, such as between the resistor material 102 and the ground plane 106. The parasitic effects may deteriorate the electrical characteristics of the embedded resistor device 100. Moreover, the parasitic effects may increase with the operating frequency of the embedded resistor device 100. In radio-frequency applications, the required impedance may be hundreds to thousands of ohms. However, the parasitic effects may reduce the actual impedance of the conventional embedded resistor device 100 to several to tens of ohms. FIG. 2 shows a diagram of the impedance magnitude of the embedded resistor device 100 illustrated in FIG. 1 at various frequencies. As illustrate in FIG. 2, the impedance may decrease as the operating frequency increases. In some applications such as radio frequency (RF) circuits, the embedded resistor device 100 may not be acceptable due to abrupt decrease in impedance.
Many embedded resistor device structures have been proposed to provide improved frequency performance. For example, U.S. Pat. No. 7,038,571 to Dunn et. al, entitled “Polymer Thick Film Resistor, Layout Cell, and Method,” and U.S. Pat. No. 5,420,562 to Kaltenecker, entitled “Resistor Having Geometry for Enhancing Radio Frequency Performance” described some embedded resistor device structures. However, conventional devices sometimes do not provide a relatively high impedance at a relatively high operating frequency or are not suitable for designs with a relatively large length/width ratio. Therefore, there may be a need for an embedded resistor device providing an improved frequency performance.